Plating solutions for rhodium and rhodium alloy platings having low internal stress



United States Patent Office U.S. Cl. 204-43 6 Claims ABSTRACT OF THE DISCLOSURE An electrolyte for electroplating low stress rhodium and rhodium alloy deposits. The electrolyte comprises a rhodium salt, optionally a salt of another metal capable of alloying with rhodium, and at least one member of the group consisting of hexametaphosphoric acid and alkali metal and ammonium hexametaphosphates.

The present invention relates to novel and improved plating solutions for plating rhodium and rhodium alloy platings.

For the formation of rhodium plating industrially, plating solutions of sulfuric acid type (rhodium sulfate-lsulfuric acid) and phosphoric acid type (rhodium phosphate+orthophosphoric acid) have heretofore been used. Rhodium alloy plating is not 'widely employed at the present time. Recently, there has been a desired for an improved rhodium plating in various industries in the light of the fact that the rhodium platings obtained with the conventional plating solutions for said platings had the drawback of the internal stress of the electrodeposition obtained being high. Additionally, with the conventional plating solutions, the surface of the resultant platings are apt to be clouded or current efiiciency is poor, that it to say that the industrial control of the plating operation is not easy.

Furthermore, in recent years, the rhodium platings are required not only to have a low internal stress, but also to have specifically improved physical properties, such as high hardness and improved wear-resistance of the surface of the platings. In order to meet such requirements, an attempt is being made, as a solution, to develop a method of electrodepositioning rhodium in the form of an alloy but such a method has not yet been practised widely industrially. In electrodepositioning rhodium in the form of an alloy between it and another metal, it is difficult to obtain a rhodium alloy plating having a uniform composition or to vary the alloy composition as desired, even by the use of a plating solution consisting of the conventional rhodium plating solution as a base and a solution having a different metal dissolved therein.

According to the present invention, there are provided novel and improved plating solutions for rhodium and rhodium alloy platings, with which the above-mentioned drawbacks can be alleviated. Namely, according to this invention, the drawbacks possessed by the conventional plating solutions can be effectively avoided by adding to a plating solution a polyphosphoric acid or a salt thereof or a mixture of said polyphosphoric acid and said salt.

The object of the invention, therefore, is to provide novel and improved plating solutions of the composition described above for rhodium and rhodium alloy platings. The rhodium platings and rhodium alloy platings produced with the conventionally known plating solutions had the tendency of being separated from the core metal or cracked due to an extremely high electro- 3,515,651 Patented June 2, 1970 deposition internal stress. In order to eliminate such undesirable phenomena, there has been proposed (1) to raise the concentration of a plating solution to be used, (2) to raise the concentration of an acid to be used, (3) to elevate the temperature of the plating solution, (4) to minimize impurities present in the plating solutions, and (5) to subject a core metal to be plated to etching. However, these methods only, though effective to some extent, are not sufliciently satisfactory. There has also been proposed a plating solution bath in which the conventional plating solution is added with magnesium sulfamate or selenic acid but again such a bath has proved only insufliciently satisfactory. The use of these plating solutions has frequently resulted in low current efliciency and clouded surface of the platings formed, and it is very difficult to remove impurities in the plating solutions during plating operation.

It is also very difficult to obtain a rhodium alloy plating having a hard and wear-resistant surface, with the use of the conventional rhodium plating solution which has a salt of different metal dissolved therein. The plating obtainable with such a plating solution is uneven and has an insuflicient hardness and wear-resistance. In addition, with such a plating solution, it is impossible to form an alloy of consistent composition or to vary the composition of the alloy as desired. For these reasons, the plating solution of the type described is rarely used for industrial purposes.

The present invention contemplates to provide novel plating solutions for rhodium and rhodium alloy platings, with which the aforementioned drawbacks of the conventional plating solutions can be markedly alleviated.

The term alloy as used herein includes alloys between rhodium and other metals, e.g. platinum, gold, silver, ruthenium, indium, palladium, iridium, nickel, cobalt, copper, tin, lea d, antimony, zinc, cadmium, tungsten, molybdenum, which are normally used for electroplating, and said alloys may be of binary or of even more elements, such as of tertiary and quaternary.

According to an aspect of the present invention, a plating solution for rhodium plating is obtained by adding to a salt of rhodium at least either one of a polyphosphoric acid and an alkali salt thereof.

According to another aspect of the invention, a plating solution for rhodium alloy plating is obtained which consists of a salt of rhodium, a salt of a metal specified above and at least either one of a polyphosphoric acid and a salt thereof.

A plating solution according to this invem'on may be acidic, neutral or basic.

The invention plating solutions are novel in respect of containing therein a phosphoric acid or a salt thereof, and have the advantage that they can be formed of any noble metal indiscriminately and yet they are capable of forming an excellent layer of plating. This advantage is brought about presumably because the polyphosphoric acid and the salt thereof form a complex salt with the metal, with the result that the concentration of metal ions is lowered and also because of the fact that the polyphosphoric acid and the :salt thereof basically act somewhat like a surface active agent.

The plating solutions for rhodium platings according to the invention consists of a rhodium salt and a polyphosphoric acid or an alkali salt thereof. Rhodium salts usable in the present invention include inorganic and organic salts, such, for example, as sulfates, chlorides, nitrates, phosphates, polyphosphates, gluconates, sorbitol salts and other Water-soluble salts. Polyphosphoric acids to be employed in the invention include polyphosphoric acids having a chain bond, such, for example, as pyrophosphoric acid, tripolyphosphoric acid, tetrapolyphosphoric acid, hexapolyphosphoric acid, and polymetaphosphoric acids having a cyclic bond, such, for example, as metaphosphoric acid, trimetaphosphoric acid, tetrametaphosphoric acid and hexametaphosphoric acid. Polyphosphates to be used in the invention include alkali salt of the polyphosphoric acids depicted above, i.e. sodium polyphosphates and potassium polyphosphates having a chain bond, such, for example, as sodium pyrophosphate, potassium pyrophosphate, sodium tripolyphosphate, potassium tripolyphosphate, sodium tetrapolyphosphate, potassium tetrapolyphosphate, sodium hexapolyphosphate, potassium hexapolyphosphate, and polymetaphosphates having a cyclic bond, such, for example, as sodium metaphosphate, potassium metaphosphate, sodium trimetaphosphate, potassium trimetaphosphate, sodium tetrametaphosphate, potassium tetrametaphosphate, sodium hexametaphosphate and potassium hexametaphosphate.

The alkali salts of the invention also include ammonium salts because ammonium salts act in the same manner as alkali salts.

In practising the present invention, a solution of the above-mentioned rhodium salts is mixed with a solution of a polyphosphoric acid or an alkali salt thereof to give a desired plating solution. Alternatively, the desired plating solution may be obtained by dissolving a solid of one component in a solution of another component. Preferably, the concentrations of the component compounds are generally in the ranges specified below.

G./liter Rhodium 0.3100 Polyphosphate -400 Polyphosphoric acid 2-400 Where it is necessary to perform plating at the optimum concentration of the plating solution for the reasons of the concentration of rhodium and other industrial plating conditions, it is recommendable to verify the plating solution by the Hull cell method prior to the plating. At any rate, the above-specified concentration ranges should be suitably selected in consideration of (1) economy, (2) lowering of electric conductivity for the determination of a lower limit and (3) lowering of the effect of the resultant plating due to increase in viscosity of the plating solution for the determination of an upper limit.

The present invention, therefore, includes the use of a mixture of a polyphosphoric acid and an alkali salt thereof for the purpose of bringing the pH of the resultant plating solution in the pH range specified above. The inventive plating solution is used for plating according to conventional methods, whereby a desired plating is obtained.

The plating solutions according to this invention have the advantages listed below:

(1) It is possible to obtain a rhodium plating which has an improved uniformity and gloss as compared with that obtainable with conventional plating solution baths, e.g. rhodium sulfate bath.

(2) It is possible to form a rhodium plating of as thick as about 5,11. without losing the inherent gloss, in contrast to conventional rhodium platings.

(3) Current efiiciency with the inventive plating solutions is higher than that with conventional plating solutions for rhodium platings. The difference therebetween is particularly remarkable where the concentration of rhodium is low.

(4) Internal stress in electrodeposits is extremely low as compared with that of conventional rhodium platings.

Now, a description will be given in detail with regard to plating solutions for rhodium alloy platings. The inventive plating solutions for rhodium alloy platings consist of a salt of rhodium, a salt of ther metal and a polyphosphoric acid or a salt thereof.

Salts of rhodium and other metals to be employed in the invention include inorganic and organic salts, e.g. sulfates, chlorides, nitrates, phosphates, polyphosphates, gluconates, sorbitol salts and mannitol salts. A poly- 4 phosphoric acid and polyphosphate to be used may be selected from the group mentioned previously.

As stated hereinbefore, ammonium salts may be included in the alkali salts to be used in the invention, since they act in the same manner as alkali salts.

In practising the present invention, a desired plating solution for rhodium alloy plating may be obtained by mixing solutions of the afore-mentioned rhodium salt and salt of other metal and a solution of a polyphosphoric acid or an alkali salt thereof. The plating solution may alternatively be formed by dissolving solid of two or one component in solutions of one or two components. Preferably, the concentrations of the components are generally in the ranges specified below:

G./liter Rhodium 0.3-100 Salt of other metal 0.05-100 Polyphosphate t 10.0-400 With the plating solution formed of the components of the concentrations specified above, a satisfactory electrodeposition can be obtained, presumably because a part or whole of the metals forms of a polyphosphate, with the result that the concentration of the metal ions is extremely lowered. Thus, the present invention include the use of a mixture of a polyphosphoric acid and an alkali salt thereof.

The plating solution defined above has the following advantageous features.

(1) The solution produces a highly uniform electrodeposition.

(2) The gloss of a plating obtained does not vary with the thickness increasing.

(3) The composition of an alloy used can be varied in a wide range according to the electroplating conditions.

(4) The plating obtained has an excellent hardness and wear-resistance.

(5) The plating obtained has a uniform alloy composition.

Examples of the inventive plating solutions for rhodium and rhodium alloy platings will now be provided hereinafter.

EXAMPLE 1.RHODIUM PLATING A solution of rhodium sulfate of a concentration of 4 g./liter as rhodium was added with a solution having dissolved therein 100 g./liter of potassium hexametaphosphate to prepare a plating solution. With this plating solution, electroplating was conducted at a pH of 5.0, a temperature of 30 C., and a current density of 8 A./dm. using platinum as an anode and a copper plate as a cathode. A glossy, uniform rhodium plating was obtained.

EXAMPLE 2.RHODIUM PLATING A solution of rhodium sulfate of a concentration of 2 g./liter as rhodium was added with a solution having dissolved therein g./liter of hexametaphosphoric acid and 50 cc./ liter of sulfuric acid to prepare a plating solution. With this plating solution, electroplating was conducted at a pH of 1.0, a temperature of 45 C., and a current density of 2 A./dm. using platinum as an anode and a brass plate as a cathode. A glossy, uniform rhodium plating was obtained.

EXAMPLE 3.-RHODIUM PLATING A solution of rhodium phosphate of a concentration of 4 g./ liter as rhodium was added with a solution having dissolved therein 70 g./ liter of trimetaphosphoric acid and 50 cc./liter of phosphoric acid to prepare a plating solution. With this plating solution, electroplating was conducted at a pH of 0.01, a temperature of 45 C., and a current density of 2 A./dm. using platinum as an anode and a copper plate as a cathode. A glossy rhodium plating of an excellent uniformity was obtained.

EXAMPLE 4.-RHODI-UM PLATING A solution of rhodium sulfate of a concentration of 6 g./liter as rhodium was added with a solution having electroplating was conducted at a pH of 6.7, a temperadissolved therein 200 g./liter of potassium tripolyphosture of 30 C. and a current density of 8.0 A./dm. using phate and 25 cc./liter of sulfuric acid to prepare a plating a stainless steel plate as an anode and a brass plate as a solution. With this plating solution, electroplating was cathode. A glossy rhodium-nickel-gold alloy was obtained conducted at a temperature of 45 C. and a current denwhich had a whitish yellow color and a hardness of sity of 7 A./dm. using platinum as an anode and a H, 270. nickellated copper plate base as a cathode. A glossy The internal stresses of the platings obtained with the rhodium plating having an excellent uniformity was obplating solutions of the compositions described in the tained, foregoing examples Were measured by a simple method EXAMPLE 5.RHODIUM-TIN ALLOY PLATING 10 descnbed A solution having dissolved therein 10 g./liter of rho- Internal stress measurmg method dium sulfate as rhodium and 50 cc./ liter of sulfuric acid A 0.08 mm. thick Phosphor bronze of a size of 130 x 5 was added with a solution having dissolved therein 100 mm. was annealed for 1 hour in an atmosphere of carbon g./liter of hexaphosphoric acid and g./liter of sodium 15 dioxide at a temperature of 600 C. and used as a specistannate to prepare a plating solution. With this plating men for electrodeposition. The specimen was placed on solution, electroplating was conducted at a temperature a 1 mm. thick vinyl chloride sheet of a size of 110 x 30 of 45 C. and a current density of 2 A-./dm. using platmm. which had been thinly coated with vaseline. Then, in'um as an anode and a nickel-plated copper plate as a the specimen was superposed by packing of 0.5 mm. in cathode. A glossy rhodium-tin alloy plating having an thickness and 110 x 30 mm. in size, made of synthetic rubexcellent uniformity was obtained. her and having the central portion cut out in a size of {EXAMPLE 6 RHODIUM NICKEL 70 x 5 mm, which was further superposed by a packing ALLOY PLATING of 1 mm. in thlckness made of a vinyl sheet and having the central portion cut out in the same size as that of the A solution having (llSSOlVCd therein g./liter Of cutout formed in thg synthetic rubber packing The enrhodium sulfate as rhodium and 25 cc./liter of sulfuric tire assembly was tightly bounded together with rubber acid was added With 3, solution having dlSSOlVCd therein bands and used as a cathode for having rhodium gleeg-fliter of tl'imetaphosphoric add and 25 liter of trodeposited thereon. Upon completion of the electronickel sulfate to P p a Plating SolutiOn. With t p a deposition, the specimen was taken out and the layer of g Solution, electroplating was conducted at a f 30 vaseline on the back side thereof was washed away with ture of 30 C. and a Current denslty 0f 5 A/dmz, 115mg a solvent. The sample plating thus obtained was subjected a platinum as an anode and grass as a cathode. A glossy to fle tio test Ih d f Y Plating having an excellent The plating was conducted in the conditions mentioned formity was obtained. below:

EXAMPLE 7.RHODIUM-GOLD ALLOY PLATING (1) Plating solution Rhodium-2 g./ liter Hex-aphosphoric acid80 g. liter Sulfuric acid-50 cc./ liter Current density2A./dm.

(3) Temperature of bath-- C.

A solution of rhodium sulfate of a concentration of 0.5 g./liter as rhodium was added with a solution having dissolved therein 100 g./liter of sodium hexametaphos- (2) phate and 10 g./liter of potassium gold cyanide to pre- 40 pare a plating solution. With this plating solution, electroa 4) Plating time2 to 15 minutes (the thickness of latin was conducted at a tem erature of C. and a g density of 2 A/dm? si platinum as an anode the resultant platings ranged from about 0.2 to about and a brass plate as a cathode. A glossy rhodium-gold alloy plating having an excellent uniformity was obtained. 45 The result of the deflection test was as follows:

Plating time Plating bath 2 minutes 8 minutes 15 minutes (ltfirtveiroitiijgnal rhodium sul- 0.9 mm. deflec- 3.0mm. deflecdeflec- 8. a OIL Inve iitive rhodium plating N o deflection 0.7 deflec- 1.0 3 1 1 11. deflecbath. tion. tion.

EXAMPLE 8.RHODIUM-GOLD-NICKEL What is claimed is:

ALLOY PLATING A plating solution for rhodium electroplating, con- A Solution having dissolved therein 4 gl/liter of sisting essentially of an aqueous solution of a rhodium dium phosphate and 5 g./liter of nickel sulfate was added salt l at least one member electfed from group with a solution having dissolved therein 100 g./liter of conslstmg hexametaphosphonc and l metal potassium tripolyphosphate and 5 g./liter of potassium and t .f 9 m whlch the gold cyanide to prepare a plating solution. With this platcinnamon 0 0 g 1S 10m g g/hter ing solution, electroplating was conducted at a temperat e concenlranon o Sal hexamelaphosp from 5 ture of 45 C. and a current density of 4 A./dm. using to 400 l l i the concentrapon of sald hexameta' platinum as an anode and a nickel-plated copper plate phosphonc zlCldls 'A base as a cathode. A glossy rhodium-gold-nickel alloy p tlng solution as descrlbed 1n clalm 1, 1 n which plating having an excellent uniformity was obtained, aid rhodium salt is selected from the group consistmg of sulfates, chlorides, phosphates, polyphosphates, gluco- EXAMPLE 9-RH nates, sorbitol salts, and nitrates.

ALLOY PLATING 3. A plating solution as described in claim 1, in which A Solution of rhodium Sulfate of a concentration f 8 said solution contains hexametaphosphoric acid and said g./liter was added with a solution having dissolved there- P P P in 40 g./liter of sodium tetrapolyphosphate, 3 g./liter of A Platlng 801110011 electfoplatlng alloys of nickel gluconate and 4 g./ liter of potassium gold cyanide to prepare a plating solution. With this plating solution,

dium and another metal, consisting essentially of a rhodium salt, a salt of another metal capable of alloying With rhodium, and at least one member selected from the group consisting of hexametaphosphoric acid and alkali metal and ammonium hexametaphosphatcs, in which the concentration of said rhodium salt is from 0.3 to 100 g./liter, the concentration of said salt of said alloying metal is from 0.05 to 100 g./liter and the concentration of said member is from 10.0 to 400 g./ liter.

5. A plating solution as described in claim 4, in which said rhodium salt and said salt of other metal are selected from the group consisting of sulfates, chlorides, nitrates, phosphates, polyphosphates, gluconates and sorbitol salts.

6. A plating solution as described in claim 4, in which said solution contains hexamctaphosphoric acid and said hexametaphosphate.

References Cited UNITED STATES PATENTS 744,170 11/1903 Darlay 204-46 XR 8 923,864 6/1909 Levy 204-46 XR 2,402,661 6/ 1946 Ohl 204-47 XR 2,451,340 10/1948 Jernstedt 20447 XR 2,577,365 12/1951 Reid 20447 2,871,171 1/1959 Atkinson 204-52 XR G. L. KAPLAN, Assistant Examiner US. 01. X.R. 

